Understanding Electricity

One of the biggest hurdles facing the would-be renewable energy system owner is understanding the terminology. When I first got interested in renewable energy I knew the terms volts, amps, watts, kilowatthours, amphours but for the life of me I couldn't have told you what any of them meant.

As I taught myself more about it I learned to make sense of it all by thinking about it like a water system. A water system has a pump, a pressure tank, water in the pipes and outlets for the water (faucets, showers, etc.) A renewable energy system is set up this way also. It will have PV panels or some other charging source (the pump), a bank of batteries (the pressure tank), current in the wires (water in the pipes) and outlets for the electricity (lights, appliances, etc., the "faucets" in an electrical system).

Just like a water system an electrical system is under pressure. The pressure in an electrical system is rated in volts. A typical renewable energy system will have panels that are rated at either 12 or 24 volts. They are actually capable of producing more than their rated voltage, as much as 20 volts for a 12 volt panel and 40 or more volts for a 24 volt panel. Like water, electricity will flow from an area of high pressure to an area of low pressure. The panels have a higher electrical "pressure" than the batteries so the energy will flow from the panels to the batteries.

Here's how that works. For a reference I'll use a typical rural well water system. The pressure tank in a well system is controlled by a pressure switch that is set for a maximum and minimum pressures. The pump (which can produce a higher pressure than the maximum pressure) runs until the maximum pressure is reached. Then as you use water the pressure in the tank drops until it reaches the preset minimum and the switch turns the pump on to repressurize the tank. This pressure range can be adjusted but could, for example, be in the 40 psi minimum to 60 psi maximum range.

The charging system and battery bank of a renewable energy system work the same way. The battery bank (the pressure tank) is "pressurized" to a maximum voltage, in our system 29.5 volts. At that point a switch (called the charge controller) turns off the electricity from the PV panels (the pump) to avoid overcharging the batteries. The minimum electrical pressure for our system is 22.5 volts, however we never let it get that low. The charge controller is "smart" enough to keep the "pressure" on whenever the voltage is below maximum and the panels are producing power.

In a renewable energy system the battery bank can wear out from overuse. Batteries, even the deep-cycle type used in renewable energy systems, don't last as long if they are fully discharged and then recharged. By keeping them as fully charged as possible their useful life is increased. The PV panels never wear out and produce power any time the sun is shining.

By designing a system where the battery bank is typically discharged no more than 25% of its capacity you can maximize the life of the batteries. This will also allow for occasional deep discharge of the battery bank to get you through extended periods of no or low power output from the charging system.

This is like buying a very large pressure tank for your water system to keep your pump from cycling on and off too often. A good rule of thumb is to size your battery bank to provide 3 - 5 days of power during those times when your charge system can't produce power (such as extended periods of cloudy weather).

If the air pressure in a water tank is pressurizing water then what is the electrical pressure in a battery pressurizing? I'm glad you asked. What is actually being stored in the batteries is current, which is expressed in amps. The amps flowing through an electrical circuit are the same as the water in the pipes of your house.

In a renewable energy system tracking the flow of amps through your system will let you know how much actual power you use and how much your panels will produce. In the next article I will discuss amps and amphours and how they relate to the wattage ratings so commonly seen on appliances, PV panels and wind generators.

All Contents © 1997
Wagonmaker Press
Thomas W. Elliot


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